1. Field of the Invention
The present invention generally concerns (i) the fabrication of thin-walled ceramic tubes, on the order of 1 millimeter to less than 100 micrometers wall thickness and from 20-100 centimeters in length, by a lamination process; and (ii) the laminate thin-wall ceramic tubes so fabricated, including tubes as may optionally be possessed of any of external wrappings/windings, thickened end regions, and/or internal features including baffles.
The present invention particularly concerns the economical automated fabrication of straight, high quality, reliable, strong and optionally feature-laden thin-walled laminate ceramic tubes, and the ceramic tubes so fabricated. The ceramic tubes so fabricated are suitably used in, inter alia, Solid Oxide Fuel Cells (SOFCs).
2. Description of the Prior Art
Ceramic tubes have found a use in the manufacture of Solid Oxide Fuel Cells (SOFCs). There are several types of fuel cells, each offering a different mechanism of converting fuel and air to produce electricity without combustion. In SOFCs, the barrier layer (the xe2x80x9celectrolytexe2x80x9d) between the fuel and the air is a ceramic layer, which allows oxygen atoms to migrate through the layer to complete a chemical reaction. Because ceramic is a poor conductor of oxygen atoms at room temperature, the fuel cell is operated at 700xc2x0 C. to 1000xc2x0 C., and the ceramic layer is made as thin as possible.
Early SOFCs were produced by the Westinghouse Corporation using long, fairly large diameter, extruded tubes of zirconia ceramic. Typical tube lengths were several feet long, with tube diameters ranging from xc2xc inch to xc2xd inch. A complete structure for a fuel cell typically contained roughly ten tubes. Over time, researchers and industry groups settled on a formula for the zirconia ceramic which contains 3 mol % Y2O3. This material is made by, among others, Tosoh of Japan as product TZ-3Y.
Another method of making SOFCs makes use of flat plates of zirconia, stacked together with other anodes and cathodes, to achieve the fuel cell structure. Compared to the tall, narrow devices envisioned by Westinghouse, these flat plate structures can be cube shaped, 6 to 8 inches on an edge, with a clamping mechanism to hold the entire stack together.
A still newer method envisions using larger quantities of small diameter tubes having very thin walls. The use of thin walled ceramic is important in SOFCs because the transfer rate of oxygen ions is limited by distance and temperature; if a thinner layer of zirconia is used then the final device can be operated at a lower temperature while maintaining the same efficiency. Literature describes the need to make ceramic tubes at 150 xcexcm or less wall thickness. These new thin-wall tubes will be seen to be the subject of the present invention.
Extrusion is the most common method for making ceramic tubes. In this approach, ceramic particles are mixed with an organic binder, often a waxy material, and the material is pressed through a circular opening. The problems with this method include (i) maintaining straightness during the firing process, (ii) obtaining thin walls with no defects, and (iii) preventing sagging of the circular cross-section into an oval shape.
Numerous patents describe methods of improving the manufacture of extruded thin-wall ceramic tubes. Continuous firing in an attempt to create long tubes has been described in U.S. Pat. No. 5,227,105. Sands, et al., describe in U.S. Pat. No. 4,395,231 the rotation of a tubular furnace as the tubular devices are passed through, whereby the speed going into the sintering furnace is faster than the speed coming out of the furnace so as to account for the shrinkage of the ceramic. In U.S. Pat. No. 4,770,631, Hell, et al., describe a method of hanging tubes vertically during sintering. In U.S. Pat. No. 5,935,513, Martreuil, et al. describe firing a ceramic tube inside of a larger ceramic support tube. Other patents, including U.S. Pat. No. 4,579,707 to Kobayashi, et al., describe methods of improving the stiffness of the un-fired tube by using a thermosetting organic binder, and then applying heat immediately after extrusion.
In efforts to make small, thin walled tubes, the extrusion process faces several challenges. One is that the tubes can warp or twist during binder removal. This problem may be due to the fact that the binders commonly used for extrusion do not maintain their strength throughout the binder removal process before sintering. Another problem relates to the production of the thin walls themselves. At a thickness of 150 xcexcm or less, a fairly small defect, such as an air bubble or a binder inclusion, can cause a defect in the final tube, creating a leak that would be considered catastrophic in a SOFC. Another practical problem with extruding thin walled tubes is that they are mechanically weaker than a thicker tube, which makes mounting difficult.
Henrik Raeder of the Center for Industrial Research in Norway has described the use of tape cast ceramics for making thin walled tubes. Tape casting involves evenly coating a horizontal surface with a ceramic slurry, drying, then removing the dried film. The slurry is prepared by dispersing ceramic in an organic binder, often a mixture of polyvinyl butyryl in solvent. Raeder described using 8 to 20 mm wide strips of tape, and winding them around alumina or glass rods. The wrapped material had an overlap of 1 to 3 mm. The diameter of the rods was 2 to 6 mm. After forming the tubes, they were slipped off the ends of the rods. Except in the areas of the seam, these tube walls were one thickness of cast ceramic, and they had trouble maintaining perfect circular form.
Two methods were used by Raeder to seal the tube along the wrapped seam. One was based on applying ethanol to the seam, which dissolved the binder and made it stick to the next layer. Another method was to apply thinned slurry to the seam, which had the advantage of both sealing the seam and coating it with additional ceramic.
The present invention contemplates a new process of fabricating thin walled ceramic tubes, particularly as are useful in Solid Oxide Fuel Cells (SOFCs). The thin-wall ceramic tubes are strong during binder removal, straight during and after firing, and of high quality without defects. The laminated thin-walled ceramic tubes are suitable for use in, among other things, fuel cells.
The preferred process of the present invention begins with very thin cast ceramic tape, preferably from 10 xcexcm to 50 xcexcm, and more preferably approximately 12 xcexcm in thickness. The tape is wrapped around a mandrel, most commonly and preferably made of steel, with enough wraps to reach the desired thickness of a tube wall. To make a ceramic tube of approximate 100 xcexcm wall thickness, approximately 10 layers of 12 xcexcm tape are around the mandrel; the resulting 120 xcexcm tube will shrink to about 100 xcexcm wall thickness during sintering. To make this thin-walled ceramic tube approximately 15 cm. in lengthxe2x80x94which is common lengthxe2x80x94one can either start with a 15 cm. wide ceramic sheet and wrap it directly around the mandrel, or start with a much longer and narrower strip of ceramic tape, wrapping the tape continuously around the mandrel in a spiral pattern to attain the desired width (tube length) and thickness.
The green ceramic tube is then laminated in a pressure laminator, preferably a hydrostatic laminator where high pressure water from, most normally, 3000 to 5000 psi is applied so as to forcibly adhere the organic binder of each ceramic layer to the next. Pressure lamination (i) links the polymer chains between each ceramic layer, (ii) cross-links the polymer chains within each ceramic layer, and, importantly, (iii) fully densifies the ceramic laminate structure by removing any air and reducing porosity.
A challenge with this xe2x80x9claid upxe2x80x9d, or xe2x80x9claminationxe2x80x9d, approach is that a laminated tube will tend to stick to the mandrel after the lamination process. To solve this problem, it is preferred to make use of a coating on the mandrel, such as, most preferably, a wax. The mandrel is prepared for the tube making operation by dipping the mandrel into hot wax (or rubbing a cold wax stick onto a heated mandrel), and then letting it cool. The coated mandrel, with its solidified coating, becomes the core of the green ceramic tube. After lamination the green tube is stuck to the mandrel, but can be removed by heating the mandrel above the melting point of the wax. Even a thin coating of wax will become liquid and will then permit the tube to be removed easily. A variation of this process would be to make the mandrel completely out of wax, but that can have the disadvantage of making it difficult to maintain tube straightness. Other materials can prospectively be used to accomplish the wax coating on the mandrel, including an ice/water combination, but are not believed to be as convenient as wax.
The advantages of the method of the present invention are numerous: a ceramic tube made of thin cast tape layers will have a maximum defect size equal to the thickness of the tape, meaning that thin walled tubes can be made with very high quality. The preferred lamination process using a preferred polyvinyl butyryl binder (PVB) produces a tube of high rigidity because the high lamination pressures (i) cross links the tape, (ii) links polymer chains between lamination layers, and (iii) removes any airxe2x80x94thus reducing porosity and increasing densityxe2x80x94compared to the characteristics of a soft and un-laminated PVB tape. The green ceramic tube, once removed from the mandrel, will not sag during bakeout and can even be fired on a flat surface.
Further in accordance with the method of the present invention, it is easy to create usefully different thicknesses along the length of the tube. This is particularly useful in providing increased thickness, and strength, to the ends of a very thin tube, at which ends the tube is held for mounting. For example, a tube of desirably thin 50 xcexcm average wall thickness might prove to be very fragile for normal mounting in a solid oxide fuel cell. However, if additional tape is wrapped at the ends of the tube, say in the last 1 centimeter at each end so as to there increase the wall thickness to several hundred microns, then the tube may be held and mounted with increased reliability, and with increased resistance to breakage. Thickening of the tube ends generally serves to make ever-thinner-walled tubes mechanically practical.
A variation on this concept of selective reinforcement of the thin-wall tube is to wrap additional material down the length of the tube, and in selected areas, as a strength-enhancing and stress-absorbing binding in order to improve the burst and/or break strength of the tube (without gross effect on oxygen migration through remaining thin-walled regions of the tube). For example, a strip of green ceramic material of many tens of centimeters length can be produced from green tape that has a 50 xcexcmxc3x9750 xcexcm section. This strip can be wrapped around the tube like as the hoops of a barrel or, preferably, in a spiral, or helix. If desired, yet another piece can be wrapped around the tube, normally in a spiral or helix of opposite handedness. These top wrap(s) give added strength to the final, fired, tube.
Still yet another feature of the ceramic tube manufacturing method of the present invention is that a varied internal form can quite readily be imparted to the tube, particularly so as to create turbulence within the tube. The method of constructing thin-walled ceramic tubes with complex internal features preferably starts with a solid mandrel that has been machined or otherwise shaped so as to create a channel down the length of the mandrel, preferably a spiral channel for all or most of the length of the tube which will be formed upon the mandrel. After the mandrel is coated with wax, a ceramic green tape is wound around the mandrel and laminated. After lamination, the tube is heated and the removed from the mandrel (and vice versa). If the mandrel""s channel is a spiral (or helical) channel, as is preferred, then the tube may be turned in the manner of a nut on a threaded screw as it is backed off the mandrel. At least the inner layers of the tape wrap will penetrate into the spiral channel of the mandrel, giving a three-dimensional form inside the tube which will serve as a partial baffle to the flow of gases longitudinally through the tube, desirably increasing turbulence in this gas flow.
The net advantage of these further improvements is to give improved strength and toughness to the thinnest of ceramic tubes, thus making possible high-efficiency, reliable and strong thin-wall ceramic tubes eminently suitable for use in a solid oxide fuel cell (SOFC). Meanwhile, automated manufacturability of the tube is retained, and tubes of usefully complex contours both external and internal may be readily and inexpensive made.
In particular, the optionally thicker tube ends permit even very thin tubes to be better and more reliably mounted into the larger mating pieces. Integral wraps or windings down the exterior length of the tube give better mechanical strength to the tube itself, while maintaining the thin walls of the tube over most of its area. Finally, features created in the interior of the tube induce turbulence in the gas flow longitudinally within the tube.
1. A Method of Making a Ceramic Tube
Accordingly, in one of its aspects the present invention is embodied in a method of making a ceramic tube of wall thickness Ttube.
The method consists of (i) wrapping green ceramic, having a thickness Twrap that thinner than is the thickness Ttube of the tube, around a mandrel a multiple n times, (ii) laminating together the n wraps of the green ceramic under pressure while still wrapped about the mandrel; and then, in either sequence, both (iii) separating the mandrel from the laminated wraps, and (iv) sintering the separated laminated wraps to produce a laminated ceramic tube of wall thickness nxc3x97Twrap=Ttube.
The wrapping is preferably to a cumulative wall thickness Ttube that is less than 1 millimeter, and that is still more preferably less than 100 micrometers. Ergo the method serves to make a thin-walled ceramic tube.
The wrapped green ceramic is preferably ceramic tape which is preferably wound around the mandrel in a spiral pattern. This preferred green ceramic tape is more preferably wound around the mandrel in spirals of complimentary right-hand, and left-hand, twist, one wound layer to the next.
The wrapped green ceramic tape is preferably of a width Wtape less than 0.20 the length Tlength of the ceramic tube that is made from laminated layers of the tape. The tape is therefor wound about the mandrel at least 1/0.2=5 times so as to form each single thickness, tube end to tube end, of each wound layer.
Likewise, the wrapped green ceramic tape is preferably of a thickness Twrap=Ttape less than 0.20 the thickness Ttube of the ceramic tube that is made from laminated layers of the tape. The resulting ceramic tube of n layers thus has at least 1/0.2=5 laminate layers.
The green ceramic may alternatively be ceramic sheet, which ceramic sheet is preferably wound radially around the mandrel in equal area sheets with each sheet substantially aligned with and continued from an earlier sheet.
In this case the wrapped green ceramic sheet is preferably of a width Wsheet equal to a length Tlength of the ceramic tube made from laminated layers of the tape. Each sheet is therefor wound straight around the mandrel to form a single thickness of each wrap.
Likewise in this case, the wrapped green ceramic sheet is preferably of a thickness Tsheet less than 0.20 the thickness Ttube of the ceramic tube made from laminated layers of the sheet. The resulting ceramic tube of n layers thus again has at least 1/0.2=5 laminate layers.
The basic method may, after the wrapping but before the laminating, optionally further include a further wrapping upon the laminate layers of the tube of at least one extra winding. This extra winding is coextensive with the body of the tube as would be a wrap layer, but is rather wrapped around the tube in the manner of a rope binding, thereby to add structural strength to the tube.
This optional further wrapping may in particular be of plural spiral, or helical, windings of ceramic tape in complimentary left-hand and right-hand spirals. This optional further wrapping may also be at end regions of the tube only, and not at the central region of the tube. When the end region(s) is (are) thickly wrapped, then the central region of the tube is thinner, being of thickness Ttube, then is are this (these) end area(s) of the tube where exist(s) additional thickness of the ceramic tape.
These optionally thickened tube end regions may be derived from 1) wraps having greater thickness Twrap as are selectively applied to end regions of the tube, or else from 2) wraps of substantially equal thickness Twrap are applied to a greater number of layers n at end regions of the tube, or from both wraps types 1) and 2), so that the wrapped laminated tube is not of equal thickness, but is thicker at end regions.
The tube may optionally have internal features, or baffles. Namely, the method may be performed using a mandrel having one or more grooves, or channels. The grooves may be circumferential, but are preferably in spiral, or helical, form. The wrapping is then of successive wraps at least the interior ones of which wraps pull tight into the spiral groove(s) of the mandrel, producing after the sintering a laminate ceramic tube having a interior bore in which is present one or more grooves in the manner of the rifling of a rifle barrel. As a variation, the mandrel, and the resulting tube, may spiral grooves that are (i) intertwined, being both of a right-hand or both of a left hand type, or that are (ii) intersecting, being of both right- and left-hand types.
The green ceramic preferably contains a cross-linkable organic binder. The laminating together under pressure then consists of laminating in a hydrostatic pressure laminator. The pressure of the laminator and the laminating is sufficient to cross-link the organic binder within the ceramic of each layer, forming linked polymer molecular chains between layers.
In the most preferred method, a releasing agent, preferably wax, is placed on the surface of the mandrel before the applying of the green ceramic tape wrap. The separating of the mandrel from the laminated wrapped tape then consists of activating the releasing agent on the surface of the mandrel (such as by heating the wax until it melts), and withdrawing the mandrel from the laminated wrapped layers of ceramic.
The invention includes both laminate ceramic tubes, and thin-walled laminate tubes particularly sized, adapted and suitable for use in fuel cells, that are produced by this method.
2. Thin-walled Laminate Ceramic Tubes with Integral Features
In another of its aspects the present invention is embodied in laminated thin-walled ceramic tubes having particular, integral, features that are not know by the inventors to have been present in the prior art.
In one embodiment the thin-walled ceramic tube has at its exterior surface an integral ceramic wrap or winding which serves to strengthen the tube against bursting and breakage while permitting that a majority of the exterior surface of the tube is neither wrapped nor wound, and in these regions the tube maintains its relatively thinner walls. This ceramic wrap or winding is preferably in one or more spirals along the length of the tube, or in a number of intersecting clockwise and counter-clockwise spirals. The external appearance of the tube is thus similar in appearance to the leg windings of a roman sandal, or the intertwined snakes of a caduceus.
In another, complimentary, embodiment the laminated thin-wall ceramic tube has at its ends increased integral thickness of ceramic. This thickness serves to strengthen the tube against end damage during mounting of the tube to any external structure at, and by, the tube""s end regions.
In still another, further complimentary, embodiment, the laminated thin-wall ceramic tube has at its interior surface an integral feature, or baffle, that serves to induce turbulence in any longitudinal flow of gases within, and along the length of, the tube. This integral interior surface feature is preferably a spiral channel along the length of the tube.
3. A Complete Fuel Cell Reaction Chamber Based on a Thin-walled Laminate Ceramic Tube
In yet another of its aspects the laminated thin-walled ceramic tubes may be used in combination as a complete reaction chamber of a fuel cell.
In the preferred method of making a tubular reaction chamber of a fuel cell from concentric ceramic tubes, lengths of thin planar green ceramicxe2x80x94either sheet or, more preferably, tape, are prepared both (i) plain and with (ii) a metallization layer upon one surface.
One or more turns of first thin planar green ceramic having a metallization surface are first wrapped around a mandrel sufficiently contiguously and extensively so as to form an uninterrupted first tubular surface.
Then one or more turns of thin planar plain green ceramic surface are second wrapped around the mandrel over the first thin planar green ceramic. This second wrapping is again sufficiently contiguous and extensive so as to form an uninterrupted tubular surfacexe2x80x94a second tubular surface.
Then one or more turns of third thin planar green ceramic having a metallization surface are third wrapped around the mandrel over the second thin planar green ceramic. This third wrapping is also sufficiently contiguous and extensive so as to again form an uninterrupted, third, tubular surface.
The first and the second and the third wrappings of green ceramic as do form the first and the second and the third tubular surfaces are then laminated together under pressure while these wrappings are still wrapped about the mandrel.
Subsequently, the mandrel is separated from the laminated wraps, and the laminated wraps sintered to produce three laminated concentric ceramic tubes each of the tubes is itself laminated.
In the composite structure the innermost first tubexe2x80x94being of the first thin planar ceramic having a metallization surfacexe2x80x94is within a next, second, tubexe2x80x94being of the second thin planar plain ceramicxe2x80x94is within a next, third and outermost, tubexe2x80x94being of the third thin planar ceramic having a metallization surface. The metallization of one of the innermost first and the outermost third tube is suitably an anode, while the metallization of the other tube is suitably a cathode of a reaction chamber of a fuel cell. The second tube is suitably the electrolyte of this fuel cell reaction chamber.
Accordingly, a tubular reaction chamber of a fuel cell has been formed from concentric ceramic tubes each of which tubes is laminated, and where the entire tubular reaction chamber is laminated.
The first wrapping is preferably of one or more turns of first thin planar green ceramic having its metallization surface to the exterior, and away from the mandrel, while the third wrapping is preferably of one or more turns of third thin planar green ceramic having its metallization surface to the interior, and towards the mandrel.
These and other aspects and attributes of the present invention will become increasingly clear upon reference to the following drawings and accompanying specification.